1. Field of the Invention
This invention relates generally to apparatus for reproducing video or other information signals recorded in a track on a record medium, for example, as in a video tape recorder in which the video signals are recorded in successive parallel tracks which are skewed or extend obliquely on a magnetic tape. More particularly, the invention is directed to an improved system or arrangement by which a magnetic head or other transducer is made to accurately track or scan the track or tracks in which the video or other information signals are recorded.
2. Description of the Prior Art
It is conventional to record video or other information signals along successive parallel tracks which are skewed or extend obliquely on a magnetic tape, for example, as in a helical scan VTR. It has been proposed, for example, as disclosed in Belgian Patent Application No. 852,715, which has been laid opened to public inspection and corresponds to U.S. patent application Ser. No. 669,047, filed Mar. 22, 1976, to provide a system by which an information or data signal transducer, for example, in the form of a rotary magnetic head, is continuously maintained in a desired position in respect to the recorded information signal track or tracks on a magnetic tape in a helical scan VTR. In such previously proposed system, the position of the information signal transducer or head relative to the track is monitored during the scanning of the track through the reproduction of the recorded information signals while a small oscillatory motion or dither is imparted to the transducer or head via its supporting element or arm which is, for example, in the form of a piezo-electric bender element or bi-morph leaf. The oscillatory motion or dither is induced in the supporting element or arm by applying to the latter a suitable drive signal which causes the transducer or fluctuate or oscillate transversely about its normal scanning path. The oscillation of the transducer introduces deviations in the envelope of the reproduced information signals resulting from the scanning of the record track. Such deviations take the form of an amplitude modulation of the envelope of the reproduced signals, with the change in magnitude of the envelope being representative of the amount of transverse displacement of the transducer from the optimum transducing or centered position with respect to the track, and with the direction of transverse displacement of the transducer from the optimum transducing position being represented by the phase of the envelope amplitude modulation at the fundamental frequency of the oscillatory motion or dither. To obtain such transducer or head position information, the modulated RF envelope signal reproduced by the transducer is applied to an amplitude modulation envelope detector which recovers the dither signal fundamental and its sidebands, whereupon the output of the envelope detector is applied to a synchronous amplitude modulation detector which detects the amplitude and polarity of the output of the envelope detector with reference to the original or constant dither or oscillation signal by which the head is simultaneously made to oscillate transversely. The synchronous amplitude modulation detector provides a tracking error signal which is added to the dither or oscillation signal to provide the drive signal for effecting oscillation of the head or transducer. Generally, the amplitude of the tracking error signal is proportional to the transverse distance from the null position of the oscillated head to the track center, while the polarity of the tracking error signal is indicative of the direction of such displacement of the null position from the track center. Therefore, the tracking error signal, when added to the dither or oscillation signal, tends to align the null position of the transducer with the center of the track.
It will be apparent that, in the above-proposed automatic head tracking system, the output of the envelope detector contains various unwanted frequency components due to mechanical vibration of the bi-morph leaf supporting the head and such unwanted frequency components adversely affect the accuracy of the tracking error signal obtained when the output of the envelope detector is compared with the constant dither or oscillation signal in the synchronous amplitude modulation detector.
The existence of problems due to mechanical vibration of the bi-morph leaf supporting the reproducing head or transducer has been recognized, for example, in U.S. Pat. No. 4,080,636. In the system disclosed in such patent, the output of the reproducing head or transducer is processed in the manner described above, that is, such output is envelope-detected and then compared with the constant dither or oscillation signal which is applied to the bi-morph leaf so as to obtain the tracking error signal which is ultimately added to the dither or oscillation signal for providing the drive signal of the bi-morph leaf. In addition to the foregoing, the above-identified patent discloses a negative feedback loop for developing an electrical damping signal which is also applied to the bi-morph leaf so as to dampen its vibrations or oscillations, particularly at the resonance frequency thereof. In the system being described, the electrical damping signal is derived from a signal generator or sensor which is integral with the bi-morph leaf for generating a signal which is representative of the instantaneous deflected position of the transducer or head, and which is converted to a transducer velocity signal by means of a differentiator. The transducer velocity signal is then passed through a low pass filter which attenuates the signals attributable to second and higher order resonance characteristics of the bi-morph leaf, and which is followed by a phase lead network operative to shift the phase of signals received from the filter so that those signals having a frequency near the resonance frequency of the bi-morph leaf will have a net phase shift of 0.degree.. Finally, the output of the phase lead network is applied to an inverting or negative feedback amplifier so as to obtain the damping signal which is added to the previously-described drive signal. Thus, the signal representative of the instantaneous deflected position of the head or transducer as obtained from the signal generator or sensor integral with the bi-morph leaf is only employed for producing the feedback or electrical damping signal by which mechanical vibration of the bi-morph leaf at its resonance frequency is damped. However, such damping action does not eliminate or correct inaccuracies appearing in the tracking error signal by reason of the fact that the latter is still derived from a comparison of the fixed dither oscillation signal with the envelope detected output of the reproducing head or transducer which contains mechanically induced vibrations or other undesired frequency components.